Porcine reproductive and respiratory syndrome virus (PRRSV) belongs to a group of mammalian arteriviruses, which also include murine lactate dehydrogenase-elevating virus, simian hemorrhagic fever virus and equine arteritis virus. The arteriviruses share important properties related to viral pathogenesis, including a tropism for macrophages and the capacity to cause severe disease and persistent infection. Clinical disease syndromes caused by infection with porcine reproductive and respiratory syndrome virus (PRRSV) were first reported in the United States in 1987 (Keffaber, 1989) and later in Europe in 1990 (Wensvoort et al., 1991). Infection with PRRSV results in respiratory disease including cough and fever, reproductive failure during late gestation, and reduced growth performance. The virus also participates in a variety of polymicrobial disease syndrome interactions while maintaining a life-long subclinical infection (Rowland et al., 2012).
Since its emergence, PRRS has become the most important disease of commercial pigs in North America, Europe and Asia, with only the continents of Australia and Antarctica free from disease. In North America alone PRRSV-related losses are estimated to cost producers $664 M each year (Holtkamp et al., 2013). In 2006, a more severe form of the disease, known as highly pathogenic PRRS (HP-PRRS), decimated pig populations throughout China. Genetic diversity has limited the development of vaccines needed to effectively control and eliminate the disease. While genetic selection for natural resistance might be an option, the results have to date been limited (Boddicker et al., 2014).
Molecular comparisons between North American and European viruses place all PRRSV isolates into one of two genotypes, Type 2 or Type 1, respectively. Even though the two genotypes possess only about 70% identity at the nucleotide level (Nelsen et al., 1999), both share a tropism for CD163-positive cells, establish long-term infections, and produce similar clinical signs.
CD163 is a 130 kDa type 1 membrane protein composed of nine scavenger receptor cysteine-rich (SRCR) domains (Fabriek et al., 2005). Porcine CD163 contains 17 exons that code for a peptide signal sequence followed by nine SRCR domains, two linker domains, also referred to as proline serine threonine (PST) domains, located after SRCR 6 and SRCR 9, and a cytoplasmic domain followed by a short cytoplasmic tail. Surface expression of CD163 is restricted to cells of the monocyte-macrophage lineage. The protein was first identified in human tissues because of its ability to bind a hemoglobin-haptoglobin (HbHp) complexes (Kristiansen et al., 2001). HbHp scavenging is a major function of CD163 and locates to SRCR 3 (Madsen et al., 2004). Metabolites released by macrophages following HbHp degradation include bilirubin, CO, and free iron. One important function of CD163 the prevention of oxidative toxicity that results from free hemoglobin (Kristiansen et al., 2001; Soares et al., 2009).
CD163, as a receptor for PRRSV, was first described by Calvert et. al. (2007). Transfection of non-permissive cell lines with CD163 cDNAs from a variety of species, including simian, human, canine, and mouse can make cells permissive for PRRSV infection (Calvert et al., 2007). In addition to CD163, a second receptor protein, CD169 (also known as sialoadhesin or SIGLEC1), was identified as being a primary PRRSV receptor involved in forming the initial interaction with the GP5-matrix (M) heterodimer, the major protein on the surface of the virion (Delputte et al., 2002). In this model, the subsequent interaction between CD163 and the GP2, 3, 4 heterotrimer in an endosomal compartment mediates uncoating and the release of the viral genome into the cytoplasm (Van Breedam et al., 2010, Allende et al., 1999). A previous model describing PRRSV infection of alveolar macrophages identified SIGLEC1 (CD169) as the primary viral receptor on the surface of macrophages; however, previous work using SIGLEC1−/− pigs showed no difference in virus replication compared to wild type pigs (Prather et a., 2013). These results supported previous in vitro studies showing that PRRSV-resistant cell lines lacking surface CD169 and CD163 supported virus replication after transfection with a CD163 plasmid (Welch et al., 2010).
Many characteristics of both PRRSV pathogenesis (especially at the molecular level) and epizootiology are poorly understood thus making control efforts difficult. Currently producers often vaccinate swine against PRRSV with modified-live attenuated strains or killed virus vaccines, however, current vaccines often do not provide satisfactory protection. This is due to both the strain variation and inadequate stimulation of the immune system. In addition to concerns about the efficacy of the available PRRSV vaccines, there is strong evidence that the modified-live vaccine currently in use can persist in individual pigs and swine herds and accumulate mutations (Mengeling et al., Am. J. Vet. Res, 60(3): 334-340 (1999)), as has been demonstrated with virulent field isolates following experimental infection of pigs (Rowland et al., Virology, 259:262-266 (1999)). Furthermore, it has been shown that vaccine virus is shed in the semen of vaccinated boars (Christopher-Hennings et al., Am. J. Vet. Res, 58(1): 40-45 (1997)). As an alternative to vaccination, some experts are advocating a “test and removal” strategy in breeding herds (Dee and Molitor, Vet. Rec., 143:474-476 (1998)). Successful use of this strategy depends on removal of all pigs that are either acutely or persistently infected with PRRSV, followed by strict controls to prevent reintroduction of the virus. The difficulty, and much of the expense, associated with this strategy is that there is little known about the pathogenesis of persistent PRRSV infection and thus there are no reliable techniques to identify persistently infected pigs.
As can be seen, a need exists in the art for the development of strategies to induce PRRSV resistance to animals.